In recent years, as a result of micropatterning of nonvolatile semiconductor memories, occurrences of the error mode such as increased resistance of word lines after product shipment have been confirmed.
When a word line is totally broken, a nonvolatile semiconductor memory cannot execute a program normally and thus can let an external controller know of such error (status fail) through a program verify operation. Therefore, the external controller can instruct a program operation again after taking steps, for example, changing the word line.
However, when a subtle increase in resistance of a word line in which, for example, the program verify operation can normally be performed, but a read operation cannot be normally performed occurs, the program verify operation terminates (status pass) and the program is judged to have been executed normally and thus, the external controller no longer holds program data. Therefore, if a read operation is performed thereafter, data cannot be read from the nonvolatile semiconductor memory and the program operation cannot be performed again either, resulting in a system error.